eg.: if you type "3+5" (without quotes), the result 8 will be shown. You can also have complex expressions like "3kg in lb", "sin (90)" etc.
Once the result is shown, you can simply press the Enter key to open the result in gnome-calculator.
And beautify the Gnome3 UI, add the menu bar back or fork an older Gnome which had still the more normal UI elements like system wide menu bar, etc or revert back to the older Unity shell (not the failed new Unity). Ubuntu 12 LTS was perfect, it all went downhill after that, with awkward decisions, and Gnome3 was never good by default.
Either make it higher or learn which ones we actually click.
Even after I move over and opened LibreOffice Calc, subsequent searches still gave me the calculator as the first entry.
I'm not exactly sure how it works, but I don't seem to have the same issue that's being mentioned.
EDIT: there surely is a library with good search algortihms to utilize for the search-box, please share =)